Consideration of information disclosed by the following U.S. Patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention.
U.S. Pat. Nos. 5,002,023 and 5,046,460 both describe a VCT system within the field of the invention in which the system hydraulics include a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position of a camshaft relative to a crankshaft in response to torque reversals experienced within the camshaft. The control system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure, P.sub.c, on one end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end which results from a compression spring that acts thereon.
U.S. Pat. No. 5,107,804 describes an alternate type of VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing which replaces the oppositely acting cylinders disclosed by the aforementioned U.S. Pat. Nos. 5,002,023 and 5,046,460. The vane is oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft in response to torque reversals. The control system of this VCT system is identical to that divulged in U.S. Pat. No. 5,002,023, using the same type of spool valve responding to the same type of forces acting thereon.
Another feature of U.S. Pat. No. 5,046,460, discussed above, is biased actuation elements. A counteracting force is applied directly to the opposed cylinders to overcome the effect of a unidirectionally acting frictional torque experienced by the camshaft during normal operation. A similar problem with rotational friction also exists with any vane-type variable camshaft timing system.
In all the systems described above, timing control is achieved in response to torque reversals, or pulses, from the camshaft generated during normal operation of the engine. However, in some engines, camshaft torque reversals are not suitable for actuation of the aforementioned hydraulic system. For example, in-line six-cylinder engines have low amplitude camshaft torque characteristics which are inadequate to actuate a variable camshaft timing system. Another example is in-line four-cylinder engines which typically operate at high speeds and generate very high frequency torque pulses to which the VCT system cannot react quickly enough to cause actuation.